Some research seems to indicate that the later all solid-state Hitachi FP-C1 and possibly the FP-7 used the same 28 pin connector. Frustratingly, neither of those models seem to have documentation online I can find as well.

More poking finds at least three remote control units at least, the RU-Z1, RV-C1 and the RU-C1(U) have the same 28 pin connector, but no promises that it will work. No biggie there. I just need the pinout.
Nothing. Zilch. Nadda. Why in the goddamn name of hell is it seemingly impossible to even find the operators manuals for these things??? Every single product I've found so far has been completely devoid of any documentation online. I can find them for sale but if it's just a passive box I don't give a damn.